The present invention relates to a semiconductor laser optical system and, more particularly, to a phase synchronous semiconductor laser optical system suitable for use in recording information such as characters on a recording medium.
As shown in FIG. 2, a phase synchronous semiconductor laser is constructed as a multi-stripe type laser diode array having a multiple hetero-junction structure in which about several to ten stripe-shaped active layers 12 are arranged in rows along a pn junction interface 10. With this structure, the active layers 12 operate in basic transverse mode, and adjacent active layers 12 are connected on the energy level. As a result, laser beams which are oscillated by each active layer are synchronous in phase, thereby providing a high output. Semiconductor lasers of the above-described type include a series of products SDL-2410 and SDL-2420 (produced by Spectra Diode Labs). It is known that a laser beam oscillated by a semiconductor laser of the above-described type provides a far field pattern in which two lobes (peaks) are formed in the direction aligned with the pn junction interface, as shown in FIG. 3. Accordingly, a laser beam which forms such two lobes in a far field pattern does not converge into a single spot, and it is therefore impossible to compose an optical system having a high level of resolution if such a laser beam is used for recording information on a recording medium. A field pattern with two lobes presents a particular problem when it is required to record information such as characters on a microfilm in the form of dots, since, in this case, a resolution level on the order of 3360 dots/7.2 mm is required, and the dots must be recorded with a very high precision.
In order to cope with this problem, it has hitherto been proposed to render such a beam useful by cutting one of the lobes by means of an optical system shown in FIGS. 4A and 4B, in Appl. Phys. Lett. 41 (12), 15 Dec. 1982. This optical system comprises a phase synchronous semiconductor laser 14, a spherical lens 15, an aperture 18 extending in the direction intersecting the pn junction interface, a cylindrical lens 20 disposed in such a manner as to condense a beam oscillated in the direction aligned with the pn junction interface, and another spherical lens 21, these members being disposed in that order. As shown in FIG. 4A, rays oscillated by the phase synchronous semiconductor laser 14 in the direction intersecting the pn junction surface are made into parallel rays by the first spherical lens 15, pass through the aperture 18 and the cylindrical lens 20, and are then converged by the second spherical lens 22 onto a recording medium 24 to form a spot. On the other hand, as shown in FIG. 4B, rays oscillated in the direction aligned with the pn junction interface are converged by the spherical lens 15 onto a focus point at the aperture 18, and a clear far field pattern is formed at the position of this focus. The aperture 18 passes only the rays that form one of the lobes and blocks the rays that form the other. The rays which have passed through the aperture 18 are made into parallel rays by the cylindrical lens 20, and are then converged by the spherical lens 22 onto the recording medium 24 to form a spot. Accordingly, a clear spot having a diameter of several .mu.m is formed on the recording medium 24.
With the above-described known optical system, however, the following problem is encountered. Since part of the beam which forms one of the lobe is cut, the intensity of the laser beam part oscillated in the direction aligned with the pn junction interface is reduced to a level of about 1/2 of that of the total laser beam part, thus resulting in a reduced efficiency. This feature of the optical system renders difficult its application to recording using a recording medium requiring a high energy for recording, such as a heat mode recording material.
Japanese Patent Laid-Open No. 98320/1987 discloses a proposal in which, after a laser beam has been collimated into a bundle of parallel rays, rays forming two lobes are separated, and the two lobes are then unified by means of a reflection mirror, a half-wave plate, and a polarization beam splitter. In practice, however, it is difficult to obtain completely parallel rays with a laser according to the above-stated proposal. If the optical path of the separated rays forming one lobe is not completely equal with that of the separated rays forming the other, then when the rays are condensed by the final lens, the position of the beam waist deviates from the optical axis. This makes it difficult to converge the beam at a high energy density.